Batesian Mimicry

Batesian mimicry, in which a benign food item looks like or behaves like a distasteful or poisonous species, and Muellerian mimicry, in which noxious animals converge on the same appearance or behavior, are important self-defenses;

From: Animal Behavior (Second Edition) , 2016

Butterflies

Philip J. DeVries , in Encyclopedia of Biodiversity (Second Edition), 2001

Glossary

Aposematic

Describing an organism that is rendered less susceptible to predation by advertising its obvious unpalatability.

Batesian mimicry

A form of mimicry in which the target organism is rendered less susceptible to predation by its resemblance in morphology or coloration to a different species that is unpalatable.

Cryptic

Describing an organism that is concealed or obscured by the similarity of its appearance to the surrounding environment.

Müllerian mimicry

A form of mimicry in which two or more unpalatable species resemble each other, with the effect that predators are more likely to avoid any species with this appearance.

Myrmecophily

Ability to form symbiotic associations with ants.

Vibratory papillae

Mobile, grooved, rod-like appendages arising from the distal edge of the first thoracic segment, used for communicating.

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Coevolution

Douglas J. Futuyma , André Levy , in Encyclopedia of Biodiversity (Second Edition), 2001

Mimicry

Another form of coevolution among prey species subject to common predators is the coevolution of mimicry. Two common forms of mimicry are typically distinguished. Batesian mimicry refers to the convergence of palatable mimic species on distasteful models. Predators learn to avoid certain prey shape and color patterns they experienced as distasteful and mimics of such patterns can profit from this aversion. Monarch butterfly larvae, Danaus plexippus, feed almost exclusively on milkweed, from which they sequester cardiac glycosides. These toxic compounds are retained in the adult and vertebrate predators quickly learn to avoid both monarch adults and the more palatable mimetic viceroy butterfly, Limenitis archippus. Cleaner fish provide a variation on Batesian mimicry. In coral reefs in the Pacific, many fish allow cleaner fish, such as the sea swallow (Labroides dimidiatus), to feed on parasites on their bodies and even in the interior of their mouths. The sabre-toothed blenny (Aspidontus taeniatus) mimics the white-and-black-striped coloration and swimming pattern of Labroides. By taking advantage of the passive behavior of fish toward the model, it is able to approach fish and bite off pieces of tissue. Labroides and Aspidontus show parallel variation in color patterns across different geographic areas, which strongly suggests that indeed the mimic is converging on the model. It is a matter of discussion, however, whether mimics will instill evolution in the model, which might be expected to evolve differences that lessen the resemblance.

Müllerian mimicry refers to the convergence toward a similar pattern among unpalatable species. Faced with several undesirable species that look alike, a predator must learn a lower number of patterns to avoid. Evolution in all prey species leads toward a common pattern, and so warrants the designation of coevolution. One of the most striking cases of Müllerian mimicry, mentioned earlier, is the convergence between the neotropical butterflies Heliconius erato and H. melpomene. Despite differences in life history, these species share a common wing color pattern that varies geographically in parallel. One of the species, H. erato, is usually the most abundant where both species co-occur, raising the possibility that parallel evolution occurred by mere convergence of the rarer H. melpomene toward a common model. However, comparison between sympatric and allopatric populations of H. erato in Central America revealed that the width of the H. erato yellow hind-wing bar converges upon that of H. melpomene when in sympatry, suggesting that both species converge on each other.

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Decision-Making and Learning: The Peak Shift Behavioral Response

S.K. Lynn , in Encyclopedia of Animal Behavior, 2010

Evolution of mimicry and crypsis

Bumblebees (Bombus impatiens ) foraging for nectar exhibited peak shift when choosing the flowers to visit (and thus pollinate). In a laboratory study implementing a Batesian mimicry system, bees were trained to forage on artificial flowers (colored paper disks) under different signal parameter sets. During training, positions of 36 S+ and S– flowers, present simultaneously, were randomized in a 6  ×   6 array on the floor of a flight cage. 'Baseline' bees received an arbitrary parameter set specifying the color and number of S+ and S− flower types, and the sugar-water reward for visiting the two flower types. Three groups of comparison bees each differed from baseline by manipulating one of the three signal parameters: increased variance of S– appearance (three S– flower colors used, whereas baseline used one), decreased relative abundance of S+ (28% of stimuli were S+ flowers, whereas baseline had 50%), and decreased reward for correct detection of S+ (33% sucrose concentration, whereas baseline used 50%). When tested on a range of nine colors (four exemplars each in random positions in the flight cage), the baseline bees exhibited peak shift relative to a control group that had received no S– training. Furthermore, as predicted by the signals approach, the comparison bees exhibited larger peak shift and area shift over and above that exhibited by the baseline bees, in accordance with the increased signal-borne risk of their training regimes. Simultaneous presentation of all test stimuli was used as a way to mitigate range effects. Also, range effects do not explain the greater shift produced by increased signal variance (which maintained the same adaptation level as the baseline condition). The results indicate that in natural situations of mimicry (two signals resembling one another) or crypsis (a signal being difficult to distinguish from noise), peak shift can influence the evolution of signaling traits.

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Moths

David L. Wagner , in Encyclopedia of Biodiversity (Second Edition), 2013

Glossary

Aposematic

Warningly colored; boldly colored, usually involving reds, oranges, or yellows, as well as black and white; less commonly with only black and white pattern elements.

Batesian mimicry

Evolutionary phenomenon whereby a palatable species comes to resemble a distasteful, toxic, or otherwise protected species and thereby gains some protection from predators.

Congener

A member of a shared genus.

Crochets

Hook-like structures at the ends of abdominal prolegs for engaging silk and other substrates.

Detritivore

An animal (larva in this article) that feeds on dead organic debris; although the term covers both plant or animal matter, in moths the term is especially apt to apply to leaf litter feeding.

Diapauses

Period of hormonally controlled inactivity generally associated with weather conditions that are unfavorable to survival or reproduction. Often induced by shortening day lengths in late summer and fall, and broken by warm temperatures in spring.

Hemolymph

Translucent yellow to green fluid that fills an insect's body, sharing functions of blood and lymphic systems, but differing in that it does not transport oxygen in most insects.

Hypermetamorphic

A life cycle that includes two or more larval forms, with each often specialized for a different feeding function (and larval ecology).

Instar

A larval stage; the first instar hatches from the egg and on molting enters the second instar. Most moths undergo five or six instars prior to pupation; as few as three and more than a dozen occur in various lineages.

Maxillae

The second pair of mouthparts, located between the mandibles and the labium; the third pair of mouthparts.

Monophyletic

A group with a single evolutionary origin that includes the common ancestor and all of that ancestor's descendants.

Mullerian mimicry

Evolutionary phenomenon whereby distasteful, toxic, or otherwise protected species come to resemble one another. In so doing the members of Mullerian mimicry complex gain by more efficiently educating local predators.

Natural group

Monophyletic; group with a common origin (i.e., having a shared common ancestor).

Oviposit

To lay an egg or ovum.

Parasitoid

Predator that lives internally or externally on its host. Parasitoids are parasite-like in that they are smaller than their hosts, feed from within or on the host's body, and often do so over a period of weeks or months, but functionally they are predators because they nearly always kill their host (prey).

Pharate

A "cloaked" or hidden stage, for example, the adult moth prior to its emergence from the pupa (which in some noctuid moths can be delayed by more than 7 months).

Pheromone

A chemical released by one individual that elicits a response in a second individual of the same species.

Polyphagous

Eating plants from more than two unrelated plant families.

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Defensive Coloration

G.D. Ruxton , in Encyclopedia of Animal Behavior, 2010

Batesian Mimicry

If the predator learns that a certain signal is associated with unattractive prey and thus avoids attacking individuals that carry that signal, then an undefended species that also carried this same signal would gain protection from predators. This is the phenomenon of Batesian mimicry . In this case, there is asymmetry in the relationship between the two species with the same signal: the defended (or otherwise unattractive) one is called the model, and its signal is copied by another undefended species, the mimic. If, while the predator is learning about the signal involved, it finds a substantial proportion of the signal-bearing individuals to be generally attractive as prey (i.e., to be mimics), then the predator will not learn to avoid bearers of this signal. Thus, we should expect Batesian mimics to be at low population density compared to their models and perhaps emerge later in a season, after the learned aversion by predators has been achieved. The more common the model is and the more unpleasant it is for the predator to attack it, the more effective the learned aversion will be and the more readily a population of mimics can be supported. The model likely pays a price for this mimicry. Even if the predator does eventually learn to avoid individuals bearing the signal, if during the learning period a small number of sampled prey individuals are actually palatable mimics, then the process of learning is likely to be slowed, and this may mean that a larger number of models experience the cost of being attacked.

If the model is disadvantaged by mimicry, why does not the model evolve as quickly away from the mimic as the mimic evolves toward it? One explanation is based on the relative success of rare mimic and rare model mutants. Any change in the mimic phenotype toward the model might provide a selective advantage (because there is an increased chance of being mistakenly misclassified as a model). In contrast, major mutants of the model species away from the mimic will not spread as rapidly because they are rare and not recognized as distasteful, and thus may face reduced fitness through higher predation risk. Even if models could readily evolve away from mimics, it is unlikely that models could ever 'shake off' mimicry completely since selection to avoid mimicry depends on the presence of a high mimetic burden in the first place. In essence, Batesian mimicry may be a race that cannot be won by models unless they adopt forms than mimics cannot readily evolve toward.

The mimicry need not be a perfect replica of the model in order to gain protection; it may just have to be similar enough to put doubt in the predator's mind. This phenomenon of imperfect mimicry can clearly be seen in hoverflies, which, although they have the distinctive colored stripe pattern of wasps, can often be readily distinguished from wasps by humans on the basis of differences in body proportions. Such imperfect mimicry may be possible when the model is particularly unpleasant for predators, making the predators much less likely to experiment with something that just might be a model.

Several Batesian mimicking species are polymorphic, with different morphs in different geographical regions mimicking different local models. This polymorphism may help to keep the local density of mimics of a particular model low in comparison to the population density of their model. Sometimes, Batesian mimicry may be limited to one sex. This dimorphism may stem from differential exposure to predators between the sexes and/or one sex having a greater need for coloration for other purposes. For example, male butterflies of such a species may have their appearance constrained by the need to use coloration to attract mates, whereas the appearance of females may be less constrained and can be mimetic of another species.

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Self-Defense

Michael D. Breed , Janice Moore , in Animal Behavior (Second Edition), 2016

10.4 Mimicry and Diversion

Unpleasant experiences and even noxious tastes have given rise to one of the most fascinating areas of mimicry because animals experience lower predation risk when they look like bad tasting, dangerous, or poisonous animals. This mimicry seems to work whether or not the mimic itself tastes bad. Henry Walter Bates, a contemporary of Darwin's, first described the mimicry that bears his name— Batesian mimicry . In this type of mimicry, a palatable prey (the mimic) is favored by natural selection if it resembles an unpalatable species (the model); it will benefit if predators learn that an animal that looks like the model is not worth eating. For this to occur, the model needs to be relatively abundant. At the other extreme, Muellerian mimicry, named after Fritz Mueller, occurs when two unpalatable species converge on an appearance; both may benefit by increasing the number of "teachers" that are informing predator-"students" about these bad-tasting prey. In sum, mimicry covers a broad range of evolutionary possibilities (Figure 10.18). Batesian mimicry, in which a benign food item looks like or behaves like a distasteful or poisonous species, and Muellerian mimicry, in which noxious animals converge on the same appearance or behavior, are important self-defenses; examples range throughout the animal world. One hypothesis for the brightly colored but nontoxic king snake is that it may mimic the highly poisonous coral snake. Both have red, yellow, and black bands in differing arrangements: "Red and yellow, kill a fellow; red and black, venom lack." Many butterflies are Batesian mimics of other butterflies, whereas yellow or orange stripes on the abdomen denote Muellerian mimicry among stinging insects.

Figure 10.18. Stinging insects and their mimics. (A) This is a yellowjacket, a wasp with a very painful sting. (B) A honeybee. (C and D) Flies that mimic stinging insects.

 Photos: (A) Whitney Cranshaw, Colorado State University, Bugwood.org; (B) Deng Xiaobao; (C) Susan Ellis, Bugwood.org; (D) Johnny N. Dell, Bugwood.org.

Key Term

Batesian mimicry is a behavior in which a benign food item (prey) looks like or behaves like a distasteful or poisonous species.

Key Term

Muellerian mimicry is a behavior in which noxious animals converge on the same appearance or behavior.

Probably every student has experienced a wasp or bee sting and learned, through that experience, to associate yellow/orange and black banding in a flying insect with pain. Birds and nonhuman mammals make the same association; this results in protection for stinging species with similar appearances (Muellerian mimicry) and for nonstinging species that look like a bee or wasp (Batesian mimicry). Color and shape tell part of the story, but for a species to be a really convincing mimic of a stinging insect, behavior comes into play. Flies that look like bees add to their deceptive story by being around flowers. Flies give themselves away to knowledgeable humans (particularly entomologists) by having a different pitch to their buzz and a propensity to hover rather than to dart when they fly. The extent to which a good morphological mimic is also a good behavioral mimic often determines its success.

Discussion Point: Batesian or Muellerian Mimics?

Experimentally, it can be difficult to discover whether a mimic is Batesian or Muellerian. 41 For many years, scientists thought that Viceroy butterflies were Batesian mimics of Monarch butterflies (Figure 10.19). More recently, studies have shown that Viceroys can be quite unpalatable and that some populations of Monarchs can be tasty to birds. There are actually many instances all along this continuum between palatable/unpalatable and unpalatable/unpalatable pairings in which mimics may be unpalatable, but not as unpalatable as models. In fact, palatability can vary even within a population, so the story in at least some cases is not as neat as that presented here. Nevertheless, mimicry does happen, and in most cases, the thing that is mimicked is the model's aposematic advertisement of unpalatability. Do your own literature search to explore this issue further, and discuss with your classmates the interaction between Batesian and Muellerian mimicry.

Figure 10.19. A monarch (left) and a Viceroy butterfly (right), long thought to be a tasty mimic of the noxious Monarch butterfly. Recent research has blurred this distinction.

 Photos: left, Jeff Mitton; right, Thomas G. Barnes/US Fish and Wildlife Service.

Sometimes animals defend themselves by imitating dangerous things. A number of caterpillars have developed an alarming resemblance to snake heads. Perhaps the most realistic example of this is the Hemeroplanes caterpillar (also known as a "viper worm"). When startled, it inflates its head and appears remarkably snake-like (Figure 10.20). The mimic octopus (see earlier description) comes to mind again. 42 When disturbed or in the presence of predators, this octopus is known to mimic other animals, such as sea snakes; tellingly, the animals it mimics are often dangerous ones. Indeed, even top predators such as sea snakes might benefit from imitating dangerous animals—in this case, imitating themselves. Despite their extremely toxic venom, they nonetheless are at risk from sharks and other large predators. When they forage and explore crevices, they must relax vigilance. During these times, they use both behavior and appearance to ward off danger. In both color and pattern, the tail of the sea snake Laticauda colubrine looks remarkably similar to the head, especially from the side. When the snake is probing crevices, it slowly twists its tail, thus offering the appearance of the head to all comers. In this way, the sea snake mitigates the reduced vigilance that frequently accompanies foraging. 43

Figure 10.20. In addition to Hemeroplanes, other caterpillars such as this spicebush swallowtail (Papilio sp.) have benefitted from looking like snakes.

 Photo: Ronald F. Billings, Texas Forest Service, Bugwood.org.

A rather dramatic octopus was recently discovered in a decidedly undramatic habitat. Indeed, the ordinary and even uniform appearance of the background (silt and sandy littoral areas near the mouths of rivers in Indonesia) may account for both the reason that the octopus remained undiscovered until 2001 and also the adaptive influences that cause the octopus's mimicry strategy to be so unusual. This mimic octopus (Thaumoctopus mimicus) mimics a variety of poisonous vertebrates, ranging from poisonous fish to sea snakes, possibly because camouflage against such a uniform background might not be highly successful. (Be ready to consider the phenomenon of mimicking dangerous animals later, in the context of deterrence.) In addition, however, when fleeing, this octopus can mimic a flounder, but then match its background (camouflage) when it stops, much like M. defilippi (see Section 10.2). Clearly, although predator avoidance strategies can be categorized for ease of organized discussion by humans, to the animal doing the predator avoidance, they blend together with remarkable effectiveness.

Mimicry extends beyond adopting another animal's appearance. Some caterpillars look deceptively like bird droppings, others like twigs, leafhoppers may look more like leaves than like insects, and a variety of treehoppers mimic thorns. Perhaps even more deceptive, some animals disguise parts of their bodies so that they resemble other parts. 44 This behavior is called diversion. For instance, some Lepidoptera (e.g., Thecla togarna) have wing markings that look like heads (complete with long "antennae") on the posterior parts of their wings. There is some debate about how redirecting attack benefits the prey, but the consensus is that the predator's attack is indeed redirected by such mimicry.

In lizards, tail-flicking may serve a similar diversionary purpose. In this case, the tail does not mimic any other part of the body, but because it moves more than the rest of the animal, it draws the predator's attention away from more vulnerable areas. If the tail is seized by the predator, the lizard can autotomize (break off) the tail and escape. 45,46 In at least one species of lizard, Acanthodactylus beershebensis, this diversion is correlated with ontogenetic changes in foraging behavior. 47 Young (<3 weeks old) lizards are active predators, moving about more than older lizards and frequenting more open microhabitats. Because of this, young lizards probably expose themselves to predators more than older lizards do, which tend toward sit-and-wait foraging behavior. The young lizards also have blue tails, which they wiggle and wave more often than adults do; the blue color of the tails fades with age, along with the wiggling and waving behavior. Thus, conspicuous tail color, conspicuous tail movement, and conspicuous foraging habits are all common in young lizards, and not common in older lizards.

Even scent mimicry is possible. It is thought that ground squirrels engage in defensive scent mimicry when they chew rattlesnake skins that have been shed and then lick their fur. 48

Of Special Interest: Mass Overwintering by Snakes

Garter snakes (Thamnophis sirtalis parietalis) overwinter in dens. When they first emerge, they are cold and sluggish. In addition, emerging females are almost immediately set upon by hordes (>100) of males in mating balls. Emerging males may mimic females, producing a pheromone that is typical of a female. It was first thought that this behavior was a reproductive tactic that allowed the female-mimicking males (called she-males) increased access to females. Further research with larger sample sizes and temperature-sensing devices revealed that this was a male strategy that could have two benefits: protection from predators in the mating ball and faster posthibernation warm-up. The reproductive males seeking females have a body temperature of over 25°C; the ground is more than 15°C cooler. Females that are the subject of male attention increase their body temperatures from 4°C to 20°C in 30   min. Males in a similar situation receive similar advantages. The she-male behavior is transient and disappears as a she-male warms up. 49

Animals may even mimic a wounded and vulnerable version of themselves. Killdeers are well known for their "broken wing" displays. In this display, a parent bird, upon spotting a predator, will move away from its nest, dragging an "injured" wing. It will do this, continuing to move just out of the predator's reach, until it is a safe distance from the nest; then it will fly away. The predator, in the meantime, has been lured away from the bird's offspring (Figure 10.21).

Figure 10.21. A killdeer feigning injury. The killdeer is well known for its broken-wing display; by pretending to be hurt and unable to flee, it lures predators away from its nest—and then flies away.

 Photo: Tracy Thomas.

Finally, some animals go so far in self-mimicking as to mimic their dead selves. This behavior is called thanatosis, from a Greek word meaning "a putting to death," and is common not only among insects but also vertebrates. Virginia opossums (Didelphis marsupialis) are perhaps most famous for this behavior and have given rise to the expression "playing possum," in reference to not only death-feigning, but almost any kind of duplicity, especially that involving health. 50 The adaptive benefits of the behavior have been rigorously tested in flour beetles. Researchers selected two strains of Tribolium beetles for long and short death-feigning episodes, and showed that after 10 generations of selection, which resulted in a clear thanatosis difference between the two lines, those selected for thanatosis were far more likely to survive the attentions of a predatory spider. 51

Of Special Interest: Thanatosis in Shakespeare—The Better Part of Valor

Humans are among the vertebrates that occasionally feign death, and in the world of theater, there are few death-feigning scenes more famous than that of Falstaff in Henry IV Part I. After escaping almost certain death by playing dead, Falstaff says that playing dead in order to live is "no counterfeit." He then goes on to advise caution, speaking one of those many phrases from Shakespeare's work that has remained in use in the English language for hundreds of years:

To die is to be a counterfeit, for he is but the counterfeit of a man who hath not the life of a man; but to counterfeit dying when a man thereby liveth, is to be no counterfeit, but the true and perfect image of life indeed. The better part of valor is discretion…. (Scene 4: 114–118)

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Darwin, Charles (Darwinism)

Michael T. Ghiselin , in Encyclopedia of Biodiversity (Second Edition), 2013

Early Darwinians

Given such a range of alternatives and the small amount of research that had been done, it makes sense that from the outset there were few Darwinians other than Darwin himself. Because he was largely responsible for getting evolutionary thinking in general accepted by the intellectual community, a lot of evolutionists who did not accept natural selection nonetheless considered themselves his followers. The degree to which Darwinism, in the sense that we use that term here, was a minority position has sometimes been exaggerated. We can identify quite a number of important contemporaries of Darwin who established successful research programs based on the study of natural selection. Foremost among these of course was Alfred Russel Wallace (1832–1913), its codiscoverer. Also very distinguished was Wallace's traveling companion, Henry Walter Bates (1825–1892), the discoverer of Batesian mimicry. Second only to Darwin in his mastery of the theory was Fritz Müller (1822–1897). He is best remembered for his discovery of Müllerian mimicry, but he also was the first to propound the idea that developmental stages may recapitulate evolutionary ones. Both Fritz Müller and his brother Hermann (1829–1883) conducted magisterial research on pollination symbiosis under Darwin's influence. It is worth emphasizing that these scientists were outstanding for their performance as naturalists in the field. The kind of research that they did has been fundamental to our understanding of biodiversity because it documents how natural selection takes place in real environments.

Darwin also had important followers whose work was more focused in the museum and the laboratory. He had a close circle of followers, botanist Joseph Dalton Hooker (1817–1911), zoologists George John Romanes (1848–1894), and John Lubbock (1834–1913). There was also August Weismann (1834–1914), whose ideas about the continuity of the germplasm made natural selection seem a much more plausible explanation for evolution than Lamarckism. Neo-Darwinism is rightly associated with the name of Weismann, whose basic position was that natural selection is not just the main but the exclusive evolutionary mechanism. Actually he admitted two minor ones that had been invoked by Darwin: sexual selection and pleiotropy.

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Population Genetics

Brian Charlesworth , in Encyclopedia of Biodiversity (Second Edition), 2013

Evolution of Close Linkage

There are two biologically important features of systems with strong epistatic selection. First of all, such selection may impose strong constraints on the degree of linkage between polymorphic loci. Suppose that the population is initially segregating for alleles A and a at one locus but is initially fixed for b at a second locus. If a mutation B arises at this locus, which interacts with the alleles at the first locus in such a way that AB is selectively favored but aB is disfavored, B may be unable to invade the population unless c is below some threshold value. Only mutations at loci that are sufficiently closely linked to the first polymorphism in the system will be able to establish subsequent polymorphisms. This process has probably been important in the evolution of some of the classic examples of "supergenes" (systems of very closely linked loci held in strong linkage disequilibrium by selection), such as Batesian mimicry in butterflies ( Ford, 1975) and sex chromosomes (Charlesworth and Charlesworth, 2010). Similarly, if ab and AB are both fitter than Ab and aB, a population fixed for ab can only evolve a two-locus polymorphism if there is a double mutation to AB and if c is sufficiently small. This has probably occurred in the evolution of meiotic drive systems, which require combinations of alleles at several loci where each allele on its own is disfavored (Charlesworth and Charlesworth, 2010).

Second, there is a selective advantage to modifier alleles that reduce the frequency of genetic recombination between the two loci, once a two-locus polymorphism has been established. If suitable genetic variation in recombination rates is available, this will eventually lead to very close linkage (Fisher, 1930). This principle has wide generality; analysis of the conditions for spread of rare modifiers of recombination rates has shown that randomly mating populations under epistatic selection generating linkage disequilibrium will always tend to evolve a closer linkage (Charlesworth and Charlesworth, 2010). Since genetic recombination is a near-universal feature of living organisms, this has led to the search for situations that promote rather than repress recombination; these mostly involve forces such as mutation, genetic drift, and environmental change that perturb populations away from their equilibria under selection alone (Charlesworth and Charlesworth, 2010)

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Differentiation

Nicholas H. Barton , in Encyclopedia of Biodiversity (Second Edition), 2013

Ecological Differentiation

A simple form of natural selection, in which each gene combination has a fixed fitness, can explain much of evolution: it accounts for differentiation between populations and species as a result of changing conditions, mutation, and random drift, and despite gene flow. However, at the very least absolute fitness (i.e., the number of offspring) must decrease with density if the population is to remain bounded. We expect the relative fitnesses of different genotypes to change with their abundance in the population: selection is in general frequency-dependent. If the reproductive success of an allele decreases as it becomes more common, then genetic diversity can be maintained. This is known as balancing selection.

Maintenance of genetic diversity by frequency-dependent selection arises from competition for different limiting resources, in just the same way as maintenance of species diversity. If the concept of a "limiting resource" is interpreted broadly, then all examples of balancing selection can be seen in this way. Evidence for a heterogeneity of resources, cited by Darwin and exploited by plant breeders, comes from the increased yield of a mixture of varieties compared with a pure stand. Here, the limiting resources may consist of different nutrients, though other mechanisms are possible. A classic example of frequency-dependent selection is Batesian mimicry, in which a palatable species avoids predation by mimicking a distasteful model species. As the mimic becomes common, predators fail to associate their pattern with unpalatability, and protection is lost. Batesian mimics such as swallowtail butterflies therefore evolve diverse mimetic patterns, each of which remains rare. The limiting resources here are the various different model species. In diploid species, variation can be maintained by heterozygote advantage. The classic example is the polymorphism for the β-hemoglobin S allele, where the heterozygote gains resistance to malaria, while the homozygote suffers from sickle-cell anemia. The relative fitnesses of diploid individuals are fixed, but from the gene's point of view, fitness decreases with frequency: a common allele is more likely to find itself in a homozygote. The limiting resources here are the two kinds of allele with which each gene can find itself paired in a diploid.

The existence of heterogeneous limiting resources is, in a broad sense, necessary for maintaining diverse species, and may be responsible for much genetic differentiation within species. However, we do not know how much genetic variation is maintained by balancing selection, as opposed to mutation and gene flow, and we do not know how far organisms respond to environmental heterogeneity by evolving diverse genotypes or diverse species. A trade-off between variation within and between species is revealed by character displacement, where coexisting species tend to diverge in morphology and behavior. The Anolis lizards of the Caribbean provide a classic example. Where one species lives on an island, it has a broad distribution of body size; where two species share an island, they take up nonoverlapping distributions, and so partition the available prey. A similar phenomenon can be seen where asexual clones compete with their sexually reproducing relatives. The freshwater fish Poeciliopsis monacha and Poeciliopsis lucida occasionally mate, and produce a parthenogenetically reproducing clone. Vrijenhoek (1994) found that in streams with many different asexual clones, there tend to be fewer of the sexual progenitor species. This suggests that the space of available ecological niches can be filled either by a set of distinct asexual clones, or by members of the genetically diverse sexual population.

If multiple forms, each exploiting a different resource, coexist in a sexual population, then matings between them may produce maladapted progeny. To make this point in another way, imagine that resource use depends on a set of continuously varying traits. If these traits are determined as the sum of effects of several genes, then sexual reproduction produces an approximately normal trait distribution. There is no reason why this should match the distribution of resources which are available. The problem can be avoided if the genetic system can somehow produce the appropriate distribution despite the random shuffling of genes that occurs with sexual reproduction. For example, swallowtail butterflies produce many distinct wing patterns, each mimicking a different distasteful model. These are determined by several genes, which are so tightly linked that they behave as a single genetic locus. If such genetic tricks do not evolve, then selection favors mating behaviors that causes like to mate with like. Ultimately, such selection could lead to complete reproductive isolation between the two forms. This splitting of a single population into two separate species in the absence of spatial separation is termed sympatric speciation. It is almost impossible to demonstrate directly, since any present pattern of divergence might have originated with some spatial isolation. However, there are several plausible cases. For example, several distinct morphs of cichlid fishes have evolved within isolated crater lakes (Schliewen et al., 2001). A key question is how often ecological differentiation amongst genotypes evolves into differentiation between species in this way.

Darwin emphasized that natural selection is driven by competition for limited resources (the "struggle for existence"). Moreover, he argued that the lack of intermediate forms – the evident clustering of organisms that makes taxonomy possible – can be explained largely by the diversifying effects of competition. There is a "division of labor" amongst organisms, which (as with market economics) results from competition between individuals rather than any optimization of species' or community productivity. Similar processes account for the differentiation of the ∼105 different genes in complex organisms. Often, one gene may acquire multiple functions. For example, arginosuccinate lyase in birds acts as a lens crystallin as well as having a catalytic function. Similarly, most of the genes responsible for the establishment of segmentation in early Drosophila development are also involved much later in specifying various organs. If by chance a multifunctional gene duplicates, then each copy may specialize to become more efficient at one of the two original functions. The sequencing of complete genomes has made clear the extent of gene duplication and differentiation: most yeast genes are members of closely related families. There are close analogies between the division of labor among different species within an ecological community, different genotypes within a population, and different genes within an organism.

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Defensive Morphology

J.M.L. Richardson , B.R. Anholt , in Encyclopedia of Animal Behavior, 2010

Overview and Introduction to Types of Morphological Defenses

Defenses against predators can be divided into primary defenses, which reduce the chances of a predator encountering, detecting, or identifying the prey, and secondary defenses, which reduce the chances of an identified prey being approached, subjugated, or consumed. Primary defenses act in predator avoidance. Secondary defenses act in predator evasion. As we discuss different types of defense strategies, it is important to bear in mind that defenses may work at both levels, and rarely do prey have only one defense mechanism.

Morphological defenses are mechanical or physical properties of an organism that may help the individual to avoid predation. Often these defenses work in conjunction with a behavioral response and may be used in other contexts as well. We begin by outlining common types of morphological defenses and then consider some examples that illustrate the inherent interplay between morphological defenses and behavior.

Crypsis

Crypsis, or camouflage, can involve background matching, disruptive coloration that obscures recognizable body parts, or masquerading as an inedible object. A classic example of selection favoring camouflage to reduce detection by predators is that of the peppered moth, Biston betularia. As the industrial revolution in Europe led to a die-off of lichen on trees, leaving a darker background on which nocturnal moths rested during the day, Kettlewell showed that an initially rare dark form of the moth increased in frequency and that diurnal bird predators more readily detected and consumed pale moths on a dark tree. Rarity may also provide crypsis. Visual predators form search images when foraging, and the rare individual has an advantage if its form falls outside the predator's search image.

Disruptive coloration can decrease the chance of identification by predators. For example, many animals have a dark patch or stripe around their eye. The eye is a readily detected feature of an individual, and thus, markings that obscure the eye can provide a substantial increase in camouflage. Similarly, patterning or projections from the body can help to disguise the body shape against the background. For moth-like artificial prey differing in color from their background, disruptive coloration around the edges of the wings (e.g., black areas that go to the edge on some part of the wing) decreases mortality by bird predators.

Some animals have body shapes, colors, and patterns that mimic inedible objects common in their environment, such as leaves or sticks. Both freshwater and saltwater neotropical fish species include some leaf mimics. Numerous examples exist of insects that have a body form that resembles twigs or leaves and that adopt body positions to further resemble twigs or leaves (e.g., praying mantids). Insects, such as caddisflies, build cases out of leaves, twigs, or, sand that provide both shelter and camouflage. Many predators rely on odor, sound, or vibrations to hunt, and prey that can smell, sound, or move in a way that matches that of something other than a prey item will presumably benefit from decreased detection and identification by a predator. Crypsis within sensory modalities other than sight are less well studied and could use more attention. More detailed discussion of crypsis is dealt with elsewhere in this volume.

Aposematism

Bright colors and pattern contrasts seen in many poisonous or venomous animals provide a morphological defense that works in conjunction with the chemical defense to prevent a predator from attacking. In animals living in an environment with a heterogeneous background, crypsis will be difficult to maintain if any movement is required. Both computer simulation studies and empirical observations support the hypothesis that for an active individual in a heterogeneous environment, selection leads to both unpalatability and aposematism. Use of toxicity and warning coloration as a defense requires that predators learn to associate the coloration with unpalatability, potentially leading to prey mortality by naïve predators.

This cost is minimized in coexisting prey that share predators by Müllerian mimicry of shared colors and patterns, to reduce the chance for any one individual of being killed by a naïve predator. This type of mimicry can lead to a striking variation in morphological patterning across a species range, with one species mimicking different coexisting congeners within its range, as seen in both Heliconius butterflies and in Asian green pit vipers.

Batesian mimicry , in which palatable species mimic the warning color patterns of unpalatable species, also occurs in groups such as hoverflies that mimic bees and wasps. Selection on such a trait is inherently frequency-dependent; if palatable mimics are too frequent, predators will kill many mimics prior to encountering an unpalatable individual and the benefit for the palatable mimic is lost. Further, the unpalatable species should experience selection to modify its warning coloration and/or pattern to allow predators to distinguish it from palatable mimics; frequency-dependent selection will, however, work against evolution of such differences at this stage as rare aposematic individuals are killed before the predator can learn to avoid them. More detailed discussion of aposematism, mimicry and toxicity is dealt with elsewhere in this volume.

Body Armor

Animals with body armor or a protective shell can respond to a predator attack by withdrawing into their shell, reducing the ability of predators to get at edible soft tissue. Evidence of the potential effectiveness of this strategy is seen among variants of a land snail found distributed among Japan and several other islands in the same region. Snails found on the same island as snail-eating snakes have a modified shell, with the shell extended into the aperture opening, changing both its shape and size. Predation trials with snail-eating snakes show that this narrowed aperture opening gives the snails a significantly increased chance of escaping a snake attack over snails of similar size with a rounded opening.

Body armor, such as the lateral plates seen on sticklebacks, can also protect an individual from injury during a predator attack. A related strategy, also used by sticklebacks as well as invertebrates such as dragonfly larvae, is to have sharp spines that will cause an attacking predator to let go prior to fatal injury occurring.

Weaponry

An animal under attack by a potential predator may go on the offensive if it has weaponry such as spines, horns, or large claws. These features may or may not have arisen through selection to avoid predation. For example, European clawed lobsters have one large crushing claw that is used in foraging and intraspecific male dominance competition, but this intact claw also significantly reduces predation. Lobsters that had lost the large claw through autotomy experienced 100% mortality when attacked by predators.

Porcupines provide an exemplary case of an animal with weapons evolved as a defense against predators. The spines of porcupines are designed such that when not erect they are flexible and not easily shed, but when erect the spines are readily released from the porcupine with a minimal amount of force on the tips. This allows the porcupine to use its tail as a weapon, slapping it against a potential attacker to inflict injury.

Weaponry may also take the form of scent or poison glands. Threatened skunks release a noxious smell to fend off potential predators, even before the predator attacks. Many insects use muscular contractions to forcefully shoot a liquid containing quinone, acetic acid, or some other noxious compound toward an approaching predator. Millipedes, moths, and toads have glands that ooze out toxins at the moment of predator contact. More detailed discussion of attacks by prey on predators is dealt with elsewhere in this volume.

Body Shape and Size

Many species have modified body shapes or size when coexisting with predators, often in conjunction with other defense mechanisms. Changes in body shape in response to a predator have been observed in many taxa and are well studied in zooplankton, insects, anuran larvae, and fish. Changes in body size alone can also act to reduce predation risk. Physid snails with a larger body size have decreased mortality from crayfish, while Daphnia evolve smaller body sizes in the presence of size-selective fish predation. Changes in body shape that enhance escape success, such as a more streamlined body in fish and relatively elongated hindlimbs in lizards, may also reflect a response to selection by predators.

Eyespots

Eyespots refer to a circular color marking on the body of an animal; while the term is convenient because to humans they resemble eyes, little evidence exists as to whether predators interpret these patterns as eyes. Eyespots are most commonly seen in lepidopterans, but also occur in other insects and some fish. Eyespots are used in multiple ways as a defense. Moths, such as the hawkmoth, with cryptic forewings will move their forewings if attacked to reveal eyespots on the hindwings that are otherwise hidden when the moth is at rest. While the predator may be startled into thinking that it is now looking at its own predator's eyes, more likely the predator is simply overwhelmed by the presentation of a large amount of new visual information to process. The brief delay in the predator's attack while processing the new information can provide the potential prey with the split second it needs to get away and resettle cryptically somewhere else.

Alternatively, eyespots may act as deflection markers, deflecting a predator's potential attack away from the most vulnerable part of the body to a less vulnerable body part. A predator that aims its attack at an eyespot on the edge of a wing may provide the moth or butterfly with an opportunity to escape with only an injured wing. Other body markings thought to act as a defensive mechanism by deflecting attack from vulnerable areas include the brightly colored tails of juvenile lizards, which are autotomized if grabbed by a predator, further increasing chance of escape. Many mollusc species similarly autotomize a tail, papillae, or siphon projections and most arthropods readily autotomize limbs.

In some fish families that are poisonous but typically remain cryptic in sandy substrate, a black eyespot on the dorsal fin appears to act as a warning signal to potential predators. Fish raise their dorsal fin to display a prominent black eyespot if disturbed.

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